The present disclosure relates to a method of providing a lithography model, and particularly to a method of calibrating a sub-lithographic assist feature (SRAF) printing model.
Images printed on a photoresist for an isolated lithographic pattern are more sensitive to focus variations than images for a dense lithographic pattern. Focus-exposure matrix (FEM) curves are thus more isofocal for dense lithographic patterns. Hence, dense lithographic patterns can be printed with a critical dimension (CD) that remains within tolerances for a broader range of defocus conditions.
Sub-resolution assist features (SRAFs) are added to mask shapes to create a denser environment for robust printing of main features. The SRAFs are not intended to be reproduced as distinct features in the photoresist, but they influence and modify the exact shape with which the main features are printed in the photoresist in the presence of the SRAFs relative to shapes that would be printed in the absence of the SRAFs. In order to avoid direct printing of the SRAFs, the size and location of the SRAFs need to be carefully optimized. If properly optimized, the SRAFs can provide benefit to the process performance of the lithographic process, for example, by increasing the depth of focus or process window, while avoiding direct printing of the SRAFs as separate but unintended patterns that could transfer to subsequent steps of the chip manufacturing process.
Optical proximity correction (OPC) photoresist models are calibrated to measurements of photoresist bottom critical dimensions (CDs) of fully resolved patterns. In case of dark field exposures where the features are trenches in the photoresist, the OPC models predict only the CDs at the bottom of the photoresist, and thus, cannot predict shallow indentation occurring at the top surface of the photoresist due to the presence of assist features in the mask. These unintended indentations induced by the presence of assist features can potentially transfer to the silicon substrate, and result in a failed chip. This shortcoming on the OPC models has become a significant yield detractor, particularly when employed with aggress inverse lithography or model based SRAF placement that rely on accurate prediction of the printing risk of SRAFs.